ISLAMABAD, April 8 (Reuters) - Pakistani opposition parties are planning to challenge a sweeping new anti-terror law in court, politicians said on Tuesday, because they fear it will legalise human rights violations.

The Protection of Pakistan Bill was passed late on Monday amid protests from members of parliament and condemnation from international rights groups such as Amnesty International.

The law grants sweeping powers of arrest and detention to Pakistan's already powerful security forces.

"This law will turn Pakistan into a police state," Shah Mehmood Qureshi, vice chairman of the Tehreek-i-Insaf party, told a news conference. "We have decided to take this law to court and challenge its anti-human rights stance."

When the National Assembly approved the bill, opposition party members tore up copies and threw them in the air and walked out of the assembly in protest.

The law permits the security forces to shoot suspects on sight, detain them at secret locations for up to 90 days and carry out raids without search warrants. The security forces can also carry out secret trials.

Lawmakers warn the law is too broad and that anger over abuses by the police and army is already fuelling growing Islamist militancy.

Qureshi, a former foreign minister, said his party had the support of the country's main Islamic party, Jamaat-e-Islami, and the Muttahida Qaumi Movement (MQM), which has held sway in the commercial hub of Karachi for decades.

The main opposition, the Pakistan People's Party, has led protests against the law in the National Assembly.

Government ministers, including the law minister and minister of information, as well as members of the ruling party's legal team, were not available for comment.

The security forces, battling increasingly violent Islamist militants as well as separatist rebels in some places, deny violating human rights.

Amnesty International said the law included no safeguards against abuse.

"Rather than addressing the real law and order failings in the country, the government is taking the easier option of giving sweeping powers to security agencies," said Mustafa Qadri, the Pakistan researcher for Amnesty International.

"These laws ... represent some of the most repressive security laws in Pakistan's history."

The law says all statements made during detention are admissible as evidence and also reverses the burden of proof, placing the onus on the accused to prove they are not guilty. It also does away with the power of high courts to review lower courts' decisions.

"Under this law, you're guilty until proven innocent. It's against the very basic principles of law," said Fawad Chaudhry, a media adviser for the Pakistan People's Party.

Prime Minister Nawaz Sharif won enough seats in parliamentary elections last May to pass unpopular laws without the support of other parties.

The law is rooted in a long-running power struggle between an activist Supreme Court and the traditionally powerful military.

Since 2008, judges have been investigating the cases of hundreds of prisoners held by security forces in secret without charge.

Human rights lawyers and relatives have been fighting for years to get information about the detainees. Most have not appeared in court charged with a crime, while others were acquitted but seized after they were freed.

The military has usually refused to produce the detainees despite the insistence of judges. The new law will give the military legal protection.

"You are now giving legal cover for things security forces are already doing, like detaining people for long periods of time with no attention to rights and declaring them terrorists with little or no proof," said senior MQM member Haider Abbas Rizvi. (Editing by Katharine Houreld and Robert Birsel)